Electrochromic window with metal grid counter electrode

ABSTRACT

An electrochromic transparency comprising an electrochromic film and an ion-conductive layer disposed between a pair of electrodes is disclosed wherein the optical properties and electrochromic efficiency are improved by means of a metal grid electrode.

BACKGROUND

This invention relates generally to the art of electrochromic cells, andmore particularly to the art of transparent electrochromic windows.

Electrochromic devices have been proposed for use in alpha-numericdisplay panels in items such as digital watches, calculators and thelike. The electrochromic image formed by the application of anappropriate voltage to an electrochromic cell persists for a usefulperiod after the activating voltage is discontinued, generally until itis erased by application of an appropriate voltage of reversed polarity.

U.S. Pat. No. 3,521,941 to Deb et al discloses an electro-optical devicehaving variable optical density useful in data display comprising a pairof transparent electrodes, and a film of transition metal compound and acurrent carrier permeable insulator disposed between the electrodes, thedevice exhibiting alternate coloration and bleaching at ambienttemperature by control of the polarity of an applied electric field.

U.S. Pat. No. 4,088,392 to Meyers discloses an electro-optical devicecomprising a pair of electrodes, and a film of a transition metalcompound and a liquid electrolyte disposed between the electrodes, thedevice exhibiting coloration and bleaching thereof by control of thepolarity of an applied electric field.

U.S. Pat. No. 4,088,395 to Giglia discloses a method for forming acounter-electrode in a variable light modulating device of theelectrochromic type. The counter-electrode is a paper composition offibrous pulp and carbon.

U.S. Pat. No. 4,116,545 to Reddy discloses an electrochromic variablelight modulating device including an improved ion conducting layercomprising a polymeric composition containing polyethylene sulfonicacid, a pigment and water. One of the electrodes comprises a glasssubstrate with an electroconductive layer of tin oxide over which isdeposited an electrochromic tungsten oxide film. The second electrode isa paper electrode comprising a web of filament permeated with carbon,and is compression bonded to tin oxide coated glass.

U.S. Pat. No. 4,174,152 to Giglia et al discloses electrochromic deviceswherein the polymeric electrolyte material is a hydrophilic copolymer ofa selected acrylate or methacrylate monomer and a selected acid groupcontaining monomer, e.g. 2-acrylamido-2-methylpropanesulfonic acid.

U.S. Pat. No. 4,335,938 to Giglia discloses electrochromic deviceshaving a layer of tungsten oxide in contact with a layer of organicelectrolyte resin comprising a hydrophilic layer of homopolymer ofpoly(2-acrylamido-2-methylpropanesulfonic acid) with electrode means forchanging electrochromic properties of the device. A conventionalelectrochromic device is described as having a transparentelectrochromic electrode comprising a glass substrate with a conductivetin oxide layer and an electrochromic, e.g. tungsten oxide, film; apigmented, ion-conducting medium layer comprising a self-supportinglayer of ion-conductive polymer having a pigment dispersed therein; andan opaque counter electrode such as carbon paper.

U.S. Pat. No. 4,361,385 to Huang et al discloses electrochromic deviceshaving a layer of electrochromic tungsten oxide in contact with apolymeric electrolyte wherein the stability and speed of the device areimproved by using a copolymer of 2-acrylamido-2 -methylpropanesulfonicacid and vinyl sulfonic acid as the polymer electrolyte. Anelectrochromic film on an electrode is prepared by evaporation of anamorphous film of tungsten oxide onto a glass substrate coated withconductive tin oxide. The polymer mixture is cast, dried and hydrated incontact with the electrochromic film, and then a second electrodeconsisting of paper-carbon is pressed against the polymer layer with asecond tin oxide coated glass plate backing the carbon-paper electrode.

U.S. Pat. No. 4,375,318 to Giglia et al discloses electrochromic deviceshaving a layer of electrochromic material, e.g. tungsten oxide, incontact with a layer of organic electrolyte resin, with electrode meansfor changing electrochromic properties of the devices by electric fieldmeans. The electrolyte layer comprises a hydrophilic layer ofhomopolymer of poly(2-acrylamido-2-methylpropanesulfonic acid) with anorganic humectant. An electrochromic device is prepared by thermalevaporation of an electrochromic tungsten oxide film onto anelectroconductive tin oxide coated glass substrate. A pigmented resinsolution is coated over the tungsten oxide and dried to form anion-conductive polymer film which is hydrated to enhance ionicconductivity. An electrically conductive paper counter-electrode ispressed against the polymer with a graphite coated stainless steelplate.

U.S. Pat. No. 4,478,991 to Huang et al discloses electrochromic deviceshaving a layer of electrochromic material, e.g. tungsten oxide, incontact with a polymeric electrolyte layer, the stability and speed ofthe device being improved by using a copolymer of2-acrylamido-2-methylpropanesulfonic acid and vinyl sulfonic acid as thepolymer electrolyte. An electrochromic film on an electrode is preparedby evaporation of an amorphous film of tungsten oxide onto a glasssubstrate coated with conductive tin oxide. The polymer mixture is castover the electrochromic film, dried to form a solid film and hydrated toenhance ion conductivity. A second electrode consisting of apaper-carbon structure is pressed against the polymer layer using aconductive tin oxide coated glass plate backing the paper counterelectrode.

U.S. Pat. No. 4,645,308 to Tracy et al discloses a solid-statetransition metal oxide device comprising a plurality of layers includingan electrochromic oxide layer and anode and cathode contacts. Colorationis actuated within the electrochromic oxide layer by application of apredetermined potential and the coloration action is adapted to sweepacross the length of the electrochromic oxide layer.

SUMMARY OF THE INVENTION

The present invention provides an electrochromic cell which istransparent, and which darkens and bleaches completely at an appliedelectrical potential of less than 1.0 volt, positive and negativerespectively. The present invention accomplishes the objective by meansof a conductive metallic mesh counter electrode which allowstransparency while insuring uniform rapid charge distribution over alarge surface area and participating in a balancing half-cell reactionat a sufficient low potential to prevent electrolysis of water andconcurrent gas evolution which occur at a higher voltage in the absenceof the metal grid.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a metal grid useful as an electrode member inaccordance with the present invention.

FIG. 2 illustrates the transmittance at 550 nanometers of anelectrochromic transparency in accordance with the present invention asa function of time over the course of darkening and bleaching cycles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conventional electrochromic cells comprise a thin film of a persistentelectrochromic material, i.e. a material responsive to the applicationof an electric field of a given polarity to change from ahigh-transmittance, non-absorbing state to a lower-transmittance,absorbing or reflecting state and remaining in the lower-transmittancestate after the electric field is discontinued, preferably until anelectric field of reversed polarity is applied to return the material tothe high-transmittance state. The electrochromic film is inion-conductive contact, preferably direct physical contact, with a layerof ion-conductive material. The ion-conductive material may be solid,liquid or gel, but is preferably a polymer layer. The electrochromicfilm and ion-conductive layers are disposed between two electrodes.

As a voltage is applied across the two electrodes, ions are conductedthrough the ion-conducting layer. When the electrode adjacent to theelectrochromic film is the cathode, application of an electric fieldcauses darkening of the film. Reversing the polarity causes reversal ofthe electrochromic properties, and the film reverts to its hightransmittance state. Typically, the electrochromic film, preferablytungsten oxide, is deposited on a glass substrate coated with anelectroconductive film such as tin oxide to form one electrode. Thecounter electrode of the prior art has typically been a carbon-paperstructure backed by a similar tin oxide coated glass substrate or ametal plate.

While this conventional electrochromic device structure might beacceptable for data displays in items such as digital watches, it is notsuitable for large transparent articles such as windows. While theopaque carbon-paper counter electrode may be replaced with a thinconductive film such as tin oxide, indium oxide or gold, these thin filmelectrodes encounter lateral electrical resistance which decreases thespeed and uniformity of charge distribution as the surface area of thedevice increases. More importantly, in electric fields of about 1 volt,half-cell reactions which result in the evolution of gas from theelectrolysis of water occur at the counter electrode, depending on thepolarity, as follows:

    ______________________________________                                        Electrode                                                                             Reaction           Standard Potential                                 ______________________________________                                        Cathode 2H.sub.2 O + 2e.sup.-  → H.sub.2 + 2OH.sup.-                                              -0.828 volts                                       Anode   2H.sub.2 O → 4H.sup.+  + O.sub.2 + 4e.sup.-                                               -1.229 volts                                       ______________________________________                                    

The hydrogen and oxygen gases produced by these reactions form bubbleswhich impair the optical properties of an electrochromic cell for use asa window.

The present invention involves the use of a metal mesh as the counterelectrode, allowing transparency while providing uniform rapid chargedistribution over a large surface area and participating in a balancinghalf-cell reaction at a lower potential which prevents electrolysis ofwater and concurrent gas evolution which would otherwise occur accordingto the following reactions, wherein x is typically up to about 0.5:##STR1##

In accordance with the present invention, instead of the hydrolysis ofwater at the counter electrode, pictured on the right above, thebalancing half-cell reaction in response to the electrochromictransition of tungsten oxide is the oxidation or reduction of the metalof the metal grid counter electrode which does not produce gas which canform bubbles and decrease the optical quality of the device.

In a preferred embodiment of the present invention, the electrochromiccell is a transparent laminate comprising two glass substrates. Oneelectrode of the cell comprises one of the glass substrates coated withan electroconductive film, preferably tin oxide having a resistivity ofabout 25 ohms per square or less. The electrochromic film, preferablytungsten oxide, is deposited over the conductive film, preferably byevaporation or sputtering to a preferred thickness of about 1000 to 4000Angstroms. The second glass substrate is preferably uncoated glass. Toform the counter electrode, a metal grid is disposed adjacent to thesecond glass substrate. A preferred metal for the grid is copper. Foroptimum optical properties, the copper grid preferably has line widthson the order of 0.0025 inch (about 0.0635 millimeter) and line spacingof about 20 lines per inch (about 8 lines per centimeter). The metalgrid pattern may be square or rectangular, but is preferably a patternof interconnected circles for optimum optical properties as disclosed inU.S. Ser. No. 43,919 filed Apr. 28, 1987, the disclosure of which isincorporated herein by reference. Preferred metal grids are produced byelectroforming as disclosed in U.S. Ser. No. 43,918, filed Apr. 28,1987, the disclosure of which is incorporated herein by reference. Theelectrochromic film/conductive film coated glass plate and the uncoatedglass plate with adjacent metal grid counter electrode are preferablyspaced about 0.030 inch (about 0.76 millimeter) apart. Disposed in thisspace is an ion-conductive material, preferably an ion-conductivepolymer as known in the art. Preferred ion-conductive polymers includehomopolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) andcopolymers of AMPS with vinyl sulfonic acid. Preferably, the metal gridcounter electrode is embedded in the ion-conductive polymer at theinterface of the polymer and the uncoated glass substrate. Electricalconnections to the electrochromic film are preferably made by means of abus bar, while connections to the metal grid counter electrode arepreferably made directly.

The cell voltage in accordance with the present invention issufficiently low so that the following electrolysis reactions of water,with concurrent evolution of gas which can form bubbles, do not occur:

    ______________________________________                                        Electrode                                                                             Reaction           Standard Potential                                 ______________________________________                                        Anode   2H.sub.2 O → 4H.sup.+  + O.sub.2 + 4e.sup.-                                               -1.229 volts                                       Cathode 2H.sub.2 O + 2e.sup.-  → H.sub.2 + 2OH.sup.-                                              -0.828 volt                                        ______________________________________                                    

Instead, the metal grid counter electrode participates in balancinghalf-cell reactions at lower absolute potentials. For a copper gridcounter electrode, the following balancing half-cell reactions occurinstead:

    ______________________________________                                        Electrode  Reaction      Standard Potential                                   ______________________________________                                        Anode      Cu → Cu.sup.+  + e.sup.-                                                             -0.521 volt                                                     Cu.sup.+  → Cu.sup.++  + e.sup.-                                                     -0.153 volt                                          Cathode    Cu.sup.++  + e.sup.-  → Cu.sup.+                                                     0.153 volt                                                      Cu.sup.+  + e.sup.-  → Cu                                                            0.521 volt                                           ______________________________________                                    

The present invention will be further understood from the description ofa specific example which follows.

EXAMPLE I

A transparent electrochromic cell is prepared using two glass cellmembers. One glass substrate is clear 3 millimeter thick float glass.The other is 5 millimeter thick float glass coated with a tin oxide filmhaving a resistivity of 25 ohms per square. The conductive tin oxideglass member functions as an electrode, with a silver frit bus barapplied about the periphery. An electrochromic tungsten oxide film,WO₃.yH₂ O, wherein y represents the extent of hydration, is depositedover the conductive tin oxide film by resistive evaporation at aninitial vacuum chamber pressure of about 4×10⁻⁶ Torr. The electrochromictungsten oxide film is deposited to a thickness of about 4000 Angstroms.The electrochromic film/conductive film coated glass member ispositioned parallel with the uncoated glass member at a spacing of 0.030inch (about 0.76 millimeter) utilizing a butyl rubber spacer. Into thissealed cell construction is cast a polymerization reaction mixturecomprising 2-acrylamido-2-methylpropanesulfonic acid, water and organicsolvent, which is cured to form an ion-conductive polymer film. Imbeddedin the polymer, adjacent to the uncoated glass substrate surface, is anelectroformed copper square grid with 0.0025 inch lines at 20 lines perinch spacing, as illustrated in FIG. 1, which functions as a counterelectrode. The electrochromic cell thus formed has a luminoustransmittance of about 70 percent at a wavelength of 550 nanometers.When an electric current is applied across the cell at a superficialcurrent density of about 0.12 milliamps per square centimeter, theelectrochromic film darkens to 20 percent transmittance in about 2minutes. When the polarity is reversed, the electrochromic film returnsto its initial transmittance in about 2 minutes as illustrated in FIG.2.

The above example is offered only to illustrate the present invention.While the above example utilizes a tungsten oxide electrochromic film,any electrochromic material may be employed, such as transition metaloxides, transition metal sulfides, transition metal oxysulfides,transition metal halides, selenides, tellurides, chromates, molybdates,tungstates, vanadates, niobates, tantalates, titanates, stannates, etc.,especially oxides, sulfides and stannates of metals of Groups IV-B, V-Band VI-B, and oxides and sulfides of Lanthanide Series metals,particularly, in addition to tungsten oxide, molybdenum oxide, titaniumoxide, vanadium oxide, niobium oxide, cerium oxide, copper stannate,cobalt tungstate and various metal molybdates, titanates and niobates.

Other electrochromic materials which reverse by short-circuiting or areeffective only at elevated temperatures may also be employed. Theion-conductive layer may be chosen to be permeable to ions other thanhydrogen, such as lithium. The metal mesh counter electrode may comprisenickel or other metals or alloys as well as the preferred copper. Themetal mesh counter electrode may be coated with another material inorder to provide a particularly desired balancing half-cell reaction,i.e. at a potential lower than that of the electrolysis of water, e.g.nickel coated with tungsten oxide or niobium oxide. While electroformingis a preferred method for producing the counter electrode, any methodwhich produces a grid with acceptable optical properties may beemployed. The electrode in contact with the electrochromic material mayalso be in the form of a metal mesh, in which case the grid patterns ofthe two electrodes may be designed to complement each other or toprovide particular optical properties. The substrate members ofelectrochromic cells may be any suitably transparent material. The scopeof the present invention is defined by the following claims.

We claim:
 1. An article of manufacture having variable transmittance inresponse to an electric field comprising:a. a transparent substrate; b.an electroconductive electrode member; c. a transparent electrochromicfilm in contact with said electroconductive electrode member; d. anion-conductive layer in contact with said electrochromic film; and e. ametal grid counter electrode in contact with said ion-conductive layersaid metal capable of balancing half cell reactions at lower absolutepotentials than the electrolysis reactions of water.
 2. An articleaccording to claim 1, wherein said transparent substrate is glass.
 3. Anarticle according to claim 1, wherein said electroconductive electrodemember is a transparent film.
 4. An article according to claim 3,wherein said film comprises a conductive material selected from thegroup consisting of metals and metal oxides.
 5. An article according toclaim 4, wherein said film comprises a conductive metal oxide selectedfrom the group consisting of indium oxide, tin oxide and mixturesthereof.
 6. An article according to claim 1, wherein saidelectroconductive electrode member is a metal grid.
 7. An articleaccording to claim 1, wherein said transparent electrochromic filmcomprises an electrochromic material selected from the group consistingof transition metal oxides, transition metal sulfides, transition metaloxysulfides, transition metal halides, selenides, tellurides, chromates,molybdates, tungstates, vanadates, niobates, tantalates, titanates andstannates.
 8. An article according to claim 7, wherein saidelectrochromic material is selected from the group consisting oftungsten oxide, molybdenum oxide, titanium oxide, vanadium oxide,niobium oxide, cerium oxide, copper stannate and cobalt tungstate.
 9. Anarticle according to claim 8, wherein said electrochromic film comprisestungsten oxide at a thickness of 1000 to 4000 Angstroms.
 10. An articleaccording to claim 1, wherein said ion-conductive layer comprises apolymer.
 11. An article according to claim 10, wherein said metal gridcounter electrode is embedded in said polymer layer.
 12. An articleaccording to claim 1, wherein said metal grid comprises a metal selectedfrom the group consisting of nickel and copper.
 13. An article accordingto claim 12, wherein said metal grid comprises copper.
 14. An articleaccording to claim 1, wherein said metal grid is an electroformed grid.15. An article according to claim 1, further comprising a secondtransparent substrate adjacent to said metal grid counter electrode. 16.An article according to claim 15, wherein said second transparentsubstrate is glass.
 17. An article according to claim 16, wherein saidmetal grid counter electrode is adjacent to said second transparentsubstrate.
 18. An article of manufacture having variable transmittancein response to an electric field comprising:a. a transparent glasssubstrate; b. an electroconductive electrode member deposited on saidsubstrate; c. a transparent electrochromic tungsten oxide film incontact with said electroconductive electrode member; d. anion-conductive polymer layer in contact with said electrochromic film;and e. a copper grid counter electrode in contact with saidion-conductive polymer layer.
 19. An article of manufacture havingvariable transmittance in response to an electric field comprising:a. atransparent substrate; b. an electronconductive electrode member; c. atransparent electrochromic film in contact with said electroconductiveelectrode member; d. an ion-conductive layer in contact with saidelectrochromic film; and e. a metal grid counter electrode in contactwith said ion-conductive layer wherein said metal grid is coated with amaterial which undergoes a half-cell reaction at a potential lower thanthat of the electrolysis of water.
 20. An article according to claim 19,wherein said material is selected from the group consisting of tungstenoxide and niobium oxide.